1. Field of the Invention
The present invention relates to a transfer device installable in an image forming apparatus and to an image forming apparatus that includes the transfer device.
2. Description of the Related Art
In color image forming apparatuses of recent years, an intermediate transfer system is being widely implemented to transfer an image on a sheet of, for example, transfer paper. In the intermediate transfer system, first, single-color toner images formed on photosensitive drums are primary-transferred and superimposed on one another on an intermediate transfer belt, which is an image carrying belt, in a primary transfer unit to obtain a full-color toner image. Then, the full-color toner image is secondary-transferred on a sheet of transfer paper in a secondary transfer unit. By implementing the intermediate transfer system, images can be formed on a variety of paper material such as thin paper, heavy paper, postcard paper, and paper envelope thereby enhancing the versatility of an image forming apparatus.
However, if a sheet having thickness above a certain level enters a secondary transfer nip in the secondary transfer unit, the constant speed of movement of the intermediate transfer belt changes for a short time. That causes distortion in an image formed by primary-transfer in the primary transfer unit.
More particularly, as shown in FIG. 23, entry of the front portion of a sheet S in a secondary transfer nip causes expansion in the secondary transfer nip corresponding to the thickness of the sheet S. Consequently, there is a sudden increase in the load on an opposite roller 116. The increase in load causes temporary deceleration in the rotating speed of the opposite roller 116. As a result, the length of an intermediate transfer belt 110 rolled at the opposite roller 116 drops below the length rolled at a driving roller 114. That causes a slack in the portion of the intermediate transfer belt 110 between the opposite roller 116 and the driving roller 114. On the other hand, the length of the intermediate transfer belt 110 rolled at a supporting roller 115 exceeds the length rolled at the opposite roller 116. That strains the portion of the intermediate transfer belt 110 between the opposite roller 116 and the supporting roller 115. In this way, the load variation caused due to the entry of the sheet S in the secondary transfer nip affects the stretched state of the intermediate transfer belt 110 and causes speed variance in the rotation thereof. If, at that time, single-color toner images formed on a plurality of photosensitive drums 104 are primary-transferred on the intermediate transfer belt 110, the transferred images get distorted due to the speed variance in the rotation of the intermediate transfer belt 110.
Meanwhile, to enhance the image quality, some image forming apparatuses include a transfer and fixing unit that not only secondary-transfers a toner image from an intermediate transfer belt on a sheet but also fixes the toner image on the image-recorded sheet. In that case too, if a sheet having thickness above a certain level enters the transfer and fixing unit, the constant speed of movement of the intermediate transfer belt decreases for a short time. That causes distortion in an image formed by primary-transfer in the primary transfer unit.
Such a problem can be solved by estimating, before a sheet enters the secondary transfer nip or the transfer and fixing unit, the timing at which the movement of the intermediate transfer belt undergoes speed variance due the entry of the sheet in the secondary transfer nip or in the transfer and fixing unit. Based on the estimated timing, a feedforward control can be performed such that, at the moment of entry of the sheet in the secondary transfer nip or in the transfer and fixing unit, the rotating speed of the intermediate transfer belt increases sufficiently to cancel out the speed variance.
Japanese Patent Application Laid-open No. 2005-107118 discloses a technology in which the time is measured from the start of registration roller clutching to the entry of a sheet in the second transfer unit. The measured time is then set as the timing for subsequent feedforward control with the start of registration roller clutching as the starting point.
However, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2005-107118, there occurs a substantial variability in the time from the start of registration roller clutching to the entry of a sheet in the second transfer unit. That causes an error in the estimated time. As a result, it becomes difficult to precisely set the timing of feedforward control. Moreover, even if the clutching time is stable, the timing of the entry of a sheet in the second transfer unit varies in practice for each sheet. Thus, accurate feedforward control is difficult to perform with the estimated time.
Meanwhile, even if an image forming apparatus implements, instead of the intermediate transfer system, a direct transfer system in which the image carrying belt is a photosensitive belt and a toner image formed on the photosensitive belt is directly transferred on a sheet in a transfer unit, the abovementioned problems occur in the transfer unit.
Moreover, the abovementioned problems occur not only when supporting members around which an image carrying belt is stretched are rollers as shown in FIG. 23, but also when the supporting members, other than a driving roller that drives the image carrying belt, are non-rotating supporting members.